1. Field of the Invention
This invention relates to a switching circuit and, more particularly, it relates to a switching circuit using a semiconductor element as a switching element and adapted to drive and control the switching element by means of a pulse signal.
2. Description of the Related Art
Switching circuits comprising a semiconductor element that operates as a switching element and is designed to be driven by a pulse signal are known. The switching circuit may typically be an FET (field effect transistor). Then, the source electrode of the FET is connected to a power source and the drain electrode thereof is connected to a load circuit while a control signal is supplied to the gate electrode thereof so that the FET is controlled for ON/OFF by the control signal. When a pulse signal is supplied as control signal, the FET is held ON to supply a voltage to the load circuit only in a pulse period.
Meanwhile, such a switching circuit is required to have good switching characteristics when the switching element of the switching circuit is driven by a pulse. Particularly, when it is driven at high speed by a pulse signal, it is necessary to make the rise time and the fall time satisfactorily short.
FIGS. 6A and 6B of the accompanying drawings show waveforms of a known switching circuit, illustrating the switching characteristics thereof. FIG. 6A shows a control signal P0 that is supplied to the control electrode of the switching element. The switching element is turned from OFF to ON when the level of the control signal P0 is raised from level Low to level High. As the switching element is held ON, a voltage is supplied to the load circuit during the ON period.
FIG. 6B shows the voltage waveform+V1 of the voltage supplied to the load circuit. While the voltage waveform+V1 rises and falls to show a waveform same as that of the control signal P0, in reality, both the rise and the fall of the switching element show a time lag due to the characteristics of the switching element. Then, there arises a problem that the waveform of the output voltage+V1 becomes blunt at the rise time and also at the fall time.
Jpn. Pat. Appln. Laid-Open Publication No. 8-46502 describes a transistor circuit showing improved rising and falling characteristics of a switching element thereof. In an embodiment of transistor circuit described in the above-cited patent document, the rise time is reduced by supplying a pulse signal to the base of the switching circuit by way of a parallel circuit of a resistor and a capacitor. In this embodiment, when the level of the pulse signal is raised from level Low to level High to turn the switching element from OFF to ON, the transistor rises very rapidly under the effect of the capacitor so that it shows a short rise time.
However, a long fall time is required to the transistor to make the switching element show bad falling characteristics when the level of the pulse signal falls from level High to level Low to turn the switching element from ON to OFF because of the large discharge time constant of the capacitor. Additionally, the above-cited patent document describes an instance of connecting two switching elements in series and driving them alternately in order to improve the rising and falling characteristics of the transistor. However, such an arrangement makes the circuit configuration a complex one.
Jpn. Pat. Appln. Laid-Open Publication No. 2002-152034 describes a switching circuit with improved falling characteristics that is realized by connecting a P-channel FET and an N-channel FET in series and driving the two FETs by means of a bipolar transistor and a differential circuit. However, the above-cited patent document does not describe any means for improving the rising characteristics. Additionally, the use of two FETs also makes the circuit configuration a complex one.
Thus, the present invention provides a switching circuit comprising a semiconductor element that operates as a switching element and showing excellent switching characteristics when driving the switching element by a pulse signal.